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The statement above is correct.
Chapter eight of the rulebook, page 36:
How Many Copies Of A Spell Can I Have In My Spellbook?
Since each spell can be used only once during the game, you may want to include additional copies of key spells. You may include up to 6 copies of each Level 1 spell in your spellbook, and up to 4 copies of all higher-level spells.